Here is a very simple, generic tutorial I wrote up for a dirty metal texture. I tried to keep it pretty easy. This is just a starting point... I'm building off this basic texture and adding things like scratches, errosion, paint and maybe some crate support bars. So if you follow along with this tutorial, be sure to keep it so you can pick up where you left off :D

Part 1

Step 1: Open a blank document. I made mine 512x512 but you can work at 256x256 if you like. Fill the background with a low-saturated color. I used a grey-green.

Step 2: Open a new document with a freebie metal texture to use as your base. Do a select all -> edit -> define pattern. Click back to the original piece and put a new layer above the green. Do an edit -> fill -> pattern.
This is probably the easiest, quickest way to start out. If you want, you can use the same texture I did:

Step 3: The texture I chose repeats pretty badly, so I want to clean that up some. Select the rubber stamp tool. Hold down alt and click a section of your image to copy from. Lift off the alt-key, and with a soft-edged brush start painting in a new area. This picks up the image from the old place and “pastes” it over into the new one. You’ll probably want to fiddle around with this to get it the way you like.

Step 4: On the metal layer, decrease the textures saturation by about 60%. I also chose to change the hue a bit and make it a little greener. Set the blending mode to “overlay” and bump the opacity down the around 90%.
Go to filter -> render -> lighting effects. Fiddle with the settings to get the effect you want, here are the settings I used:

Step 5: At this point I usually run a noise filter just to make it a little grungier. Be sure to select monochromatic. After doing this increase the contrast of the texture by about 40% to make it pop a little more (or you can just duplicate the layer to get a similar effect)
Now you should have a pretty generic dirty metal texture to work with.

Part 2

Step 6: Open up the texture from the last part of the tutorial. Go ahead and flatten all the layers. Here’s a way to make reeeaaally easy scratches. Click the pencil tool and draw a buncha random lines to look like scratch marks, I used a medium grey color.

Step 7: Duplicate the layer and set it to multiply. Click back to the original and nudge it down and over 1 pixel.

Step 8: Drop the opacity of the original layer down to about 70%. If necessary make the top layer a little darker. Take the eraser tool and wear down the edges of the scratches just a bit, so it’s not so harsh. Run a noise filter on the lines if you want. :)

Part 3

Step 9: Here’s one way to apply slight corrosion and rust. Open up your original metal texture. If necessary, flatten the layers. Duplicate the background and flip it vertically. Set it to “overlay”.

Step 10: Select the eraser tool and hack off a good 2/3’s of the top layer. Be sure to follow up with a “grungey” shaped brush to make the edges uneven. Reduce the opacity to about 75% and up the contrast about 65%

Step 11: Duplicate the grunge layer, flip it horizontally and set the blend mode to “color dodge”. You may want to darken the bottom edge of this layer as well… paint it, burn it, whatever floats your boat. :)

Part 4

Step 12: Open up your original metal texture. If necessary, flatten the layers. On the layer above it, put whatever image/texture you’d like to have on the texture. I’m just using a dingbat. Set the layer to “overlay” and decrease the layer opacity to about 85%.

Step 13: Duplicate the image layer and set it to invisible. On the original (the bottom layer), do a motion blur at a 90 degree angle. Rotate the image 90 degrees counter clockwise. Go to filter -> stylize -> wind. I do the wind one. Run it twice., I did a stagger the second time.

Step 14: Rotate it back 90 degrees clockwise and turn the top layer back on. Line them up with one another if you have to. You’ll probably barely be able to see the “wind” layer so adjust the brightest and contrast as necessary. I also set the layer mode to color burn and reduce the opacity to around 20%.

Step 15: Underneath both of these layers, create a new one. Select a nice rusty color. Randomly splotch a few places at the bottom edges of your decal. Set this layer to “overlay” and reduce the opacity to about 65%. With a grungey shaped smudge tool, click on each of your rust splotches and drag downwards to create a streak. Do this all the way around and adjust as you like. It also nice to smudge a little on the decal layer and the “wind” layer. Tack on the other effects if you want :D.

This is a texture I created using several of the afore mentioned techniques. Just to kinda show what you can do with it... :)